A reluctance type motor is advantageous not only in that it produces a large output torque but also in that it does not require a magnet rotor. The reluctance type motor, however, many disadvantages too. Thus, there are few cases that the reluctance type motor has been utilized in a practical aspect.
On the other hand, a stepping motor is capable of producing a large output, but is used only for special purposes due to the reason that its stepping speed is too slow. For the DC motor, however, there is a case that a high-speed DC motor has been developed, but it cannot be widely used due to the reason that its efficiency is apt to deteriorate.
First problem for reluctance type motor includes that the switching elements are connected to both ends of respective armature coils for activating these armature coils, and this requires a number of expensive power elements, entailing the increase of the cost, and that the switching elements provided on the sides of a positive terminal of an electric power source are required to have electric signals inputted from another electric power source for controlling currents supplied to the armature coils, another factor of cost increase.
As a second problem, a reluctance type motor has a rotor provided with many salient poles, entailing the increase of the inductance. This increase of inductance causes the increase of the magnetic energy stored into or discharged from magnetic poles and salient poles, as well as the increase in the number of times of such storing and discharging cycles during one complete revolution of the rotor.
Accordingly, a reluctance type motor is advantageous in that it is capable of producing a high output torque but disadvantageous in that it is unable to rotate at high speeds. A DC motor also have the same problem if required to rotate at a higher speed.
As a third problem, since the inductance of the armature coil is very large in the case of a motor having a large output, not only a current builds up slowly at an initial stage of the current supply period but also it trails off at a terminating stage of the current supply period. The former becomes a problem in that it decrease an output torque, and the latter also becomes a problem in that it causes a counter torque to generate.
If a high-voltage is applied from an electric power source in order to make rapid the building-up of the armature current in the initial stage of the current supply section, the building-up of the armature current becomes very rapid after a magnetic saturation point. For this reason, the motor causes vibrations and electric noises, and, since the torque is small during the above-described building-up section of the armature current, there occurs a problem such that merely disadvantages are intensified.
Thus, there was a problem such that a high-speed rotation (i.e. several 10 thousand rpm) cannot be realized due to the generation of the above-described torque reduction and counter torque. Even if the rotational speed is reduced down to a generally utilized speed level (i.e. several thousands rpm), the torque reduction and the counter torque are still generated to deteriorate the efficiency of the motor. If a means for increasing an electric power source voltage is adopted in order to increase the output torque, the voltage needs to be increased to more than 1000 volts to lose a practical merit of such a system.
A brushless DC motor will be encounter with the same problems if required to be driven within a higher speed region.